A highly → magnetized vacuum
behaving as a prism for the propagation of light,
as predicted by → quantum electrodynamics (QED).
Attempts to detect this phenomenon in the laboratory have not yet succeeded
in the 80 years since it was predicted (Heisenberg & Euler,
1936, Z. Physik, 98, 714). This effect can be detected only in the presence of
enormously strong → magnetic fields,
such as those around → neutron stars.
Owing to the large inferred magnetic fields (B ~ 1013 G,
→ gauss),
radiation from these sources is expected to be substantially polarized,
independently of the mechanism actually responsible for the
→ thermal emission. The strongest magnetic field so far
created in a laboratory is less than 106 G lasting only for several tens
of milliseconds. A large observed → polarization degree
is, however, expected only if QED polarization effects are present in the
magnetized vacuum
around the star. The detection of a strongly → linearly polarized
signal would therefore provide the observational evidence
of QED effects in the strong-field regime.
Recently a team of astrophysicists (Mignani et al. 2016, arXiv/1610.08323) have
detected → linear polarization
toward the neutron star RXJ1856.5-3754 (at a significant degree of around 16%).
This finding is likely due to the boosting effect of vacuum
birefringence occurring in the area of empty space surrounding the neutron star.

In particle physics the lowest energy allowed by field quantization when all fields are
in their → ground states. Vacuum energy is predicted to arise from
→ virtual particles that fluctuate in and out of existence,
as manifested by the → Casimir effect.
The cosmological → dark energy is postulated to be related
to vacuum fluctuations. There is however an enormous discrepancy with the predictions of
quantum field theory. In this theory the value of vacuum energy density is expected
to be roughly of the order
ρv≅ Emax4,
where Emax is the maximum energy at which the field theory is valid.
At energies of the order of the
→ Planck energy,
EPl≅ 1019 GeV, vacuum energy might be roughly:
ρv≅ EPl4≅ 1076 GeV4.
On the other hand, the vacuum energy density in standard cosmological model
is given by:
ρΛ = ΩΛ.ρcrit, where
ΩΛ is the → density parameter for the
→ cosmological constant and ρcrit
is the → critical density. More explicitly,
ρΛ = ΩΛ . 3 H2/(8πG).
Using
present-day values of ΩΛ (0.7) and H (70) leads to
ρΛ = 10-46 GeV4. Therefore,
the discrepancy between the prediction and the observed value is 122 orders
of magnitude.

A quantum field theory a process in which an electromagnetic field
gives rise to virtual electron-positron pairs that in turn exert electromagnetic
fields of their own, in a manner similar to classical dielectric polarization.